1. Field of the Invention
The present invention relates to an automatic transmission which transmits rotation of an input shaft to an output member via a plurality of planetary gear mechanisms arranged concentric to the input shaft and by carrying out gear changes in a plurality of gears.
2. Description of the Related Art
Conventionally, as this type of an automatic transmission, there is known a one from the Japanese Patent Laid-Open No. 2003-240068 (FIG. 4) which uses three, that is, a first to third planetary gear mechanisms arranged concentric to the input shaft and five engagement mechanisms in order to carry out gear changes in six speeds in the forward direction.
In this type of automatic transmission, the first planetary gear mechanism and the second planetary gear mechanism are configured from a single pinion type. The third planetary gear mechanism is configured from a double pinion type. And, a sun gear of the first planetary gear mechanism is coupled to the input shaft.
A ring gear of each of the second planetary gear mechanism and the third planetary gear mechanism are configured from the same member. A pinion which is supported by a carrier of the second planetary gear mechanism is configured from one of a pair of pinions of the third planetary gear mechanism. The carrier of the second planetary gear mechanism and the carrier of the third planetary gear mechanism are coupled together. By coupling the ring gears together and the carriers together, the second and the third planetary gear mechanisms configure a compound planetary gear mechanism. The compound planetary gear mechanism is equipped with four, that is, a first to fourth rotational elements.
The first rotational element is configured from a sun gear of the third planetary gear mechanism. The second rotational element is configured from the ring gear of the second planetary gear mechanism which is common with the ring gear of the third planetary gear mechanism. The third rotational element is configured from the carrier of the second planetary gear mechanism and the carrier of the third planetary gear mechanism which are coupled together. The fourth rotational element is configured from the sun gear of the second planetary gear mechanism.
The first through fourth rotational elements are arranged in order with spaces corresponding to gear ratios in a velocity diagram therebetween. And, the first rotational element is coupled to a carrier of the first planetary gear mechanism, and the third rotational element is coupled to the output member.
Further, the automatic transmission is equipped with, as an engagement mechanism, a total of five engagement mechanisms, namely, two clutches, those are, a first clutch and a second clutch, and three brakes, those are, a first through third brakes. The first clutch couples the fourth rotational element and the input shaft releasably. The second clutch couples the second rotational element with the input shaft releasably. The first brake fixes the first rotational element to the transmission case releasably. The second brake fixes the second rotational element to the transmission case releasably. The third brake fixes the ring gear of the first planetary gear mechanism to the transmission case releasably.
With the above configuration, a first gear speed is established by engaging the first clutch and the second brake. A second gear speed is established by engaging the first clutch and the first brake. A third gear speed is established by engaging the first clutch and the third brake. A fourth gear speed is established by engaging the first clutch and the second clutch. A fifth gear speed is established by engaging the second clutch and the third brake. A sixth gear speed is established by engaging the second clutch and the first brake.